A filter is a common device in the field of signal processing, through which signals within a particular frequency range can be passed, while other signals outside the particular frequency range will be blocked. However, in fact, the frequency range that is allowed to pass through a corresponding filter is fixed, that is, each filter corresponds to a fixed frequency range, and if the frequency ranges of an input signal are not fixed, that is, the input signal includes a plurality of frequency ranges, it may need more than one filter to serve for the input signal. When the frequency ranges of the input signal are changed, that is, when the frequency range of the input signal is changed from one to another, it needs to switch the filter from one to another correspondingly. Referring to FIG. 1, it is assumed that a desired output signal would be an output signal as shown in FIG. 1 after an input signal is filtered by two filters. However, due to an effect of switching instant, an output value of the filter at the switching instant and output values of the filter before and after the switching instant are discrete, which may lead to distortion of the output signal. Thus, the actual output signal may be the distorted output signal after the switching as shown in FIG. 1.